Planet Formation: How Ocean Worlds Happen

byPaul GilsteronFebruary 14, 2019

It’s hard to fathom when we look at a globe, but our planet Earth’s substantial covering of ocean is relatively modest. Alternative scenarios involving ‘water worlds’ include rocky planets whose silicate mantle is covered in a deep, global ocean, with no land in sight. Kilometer after kilometer of water covers a layer of ice on the ocean floor in these models, making it unlikely that the processes that sustain life here could develop — how likely is a carbon cycle in such a scenario, and without it, how do we stabilize climate and make an inhabitable world?

These are challenging issues as we build the catalog of exoplanets and try to figure out local conditions. But it’s also intriguing to ask what made Earth turn out as dry as it is. Tim Lichtenberg developed a theory while doing his thesis at the Eidgenössische Technische Hochschule in Zürich (he is now at Oxford), and now presents it in a paper in collaboration with colleagues at Bayreuth and Bern, as well as the University of Michigan. Lichtenberg thinks we should be looking hard at the radioactive element Aluminium-26 (26Al).

Go back far enough in the evolution of the Solar System and kilometer-sized planetesimals made of rock and ice moved in a circumstellar disk around the young Sun, eventually through the process of accretion growing into planetary embryos. In this era a supernova evidently occurred in the astronomical neighborhood, depositing 26Al and other elements into the mix. Using computer simulations of the formation of thousands of planets, the researchers argue that two distinct populations emerge, water worlds and drier worlds like Earth.

“The results of our simulations suggest that there are two qualitatively different types of planetary systems,” says Lichtenberg: “There are those similar to our Solar System, whose planets have little water. In contrast, there are those in which primarily ocean worlds are created because no massive star, and so no Al-26, was around when their host system formed. The presence of Al-26 during planetesimal formation can make an order-of-magnitude difference in planetary water budgets between these two species of planetary systems.”

Image: Planetary systems born in dense and massive star-forming regions inherit substantial amounts of Aluminium-26, which dries out their building blocks before accretion (left). Planets formed in low-mass star-forming regions accrete many water-rich bodies and emerge as ocean worlds (right). Credit: Thibaut Roger.

Because planets grow from these early planetesimals, their composition is critical. If a great part of a planet’s water comes from them, then the danger of accreting too much water is always present if many of the constituent materials come from the icy regions beyond the snowline. But radioactive constituents like 26Al inside the planetesimals can create heat that can evaporate much of the initial water ice content before accretion occurs. Dense star-forming regions are more likely to produce planets that manifest these latter outcomes.

Lichtenberg and team examined the decay heat from 26Al in terms of this early planetesimal evolution, which would have led to silicate melting and degassing of primordial water abundances. Their simulations of planet populations delved into internal structures that varied according to disk structures, planetary composition, and initial location of planetary embryos. They produced statistical variations of incorporated water in planets that varied in radius and initial 26Al abundance. In all, the authors achieved what they believe to be a statistically representative set of 540,000 individual simulations over 18 parameter sets.

We wind up with planetary systems with 26Al abundances similar to or higher than the Solar System forming terrestrial planets with lower amounts of water, an effect that grows more pronounced with distance from the host star, since embryos forming there are likely to be richer in water. Systems poor in 26Al are thus far more likely to produce water worlds. A remaining question involves the actual growth of rocky planets, as the paper notes:

If rocky planets grow primarily from the accumulation of planetesimals, then the suggested deviation between planetary systems should be clearly distinguishable among the rocky exoplanet census. If, however, the main growth of rocky planets proceeds from the accumulation of small particles, such as pebbles, then the deviation between 26Al-rich and 26Al-poor systems may become less clear, and the composition of the accreting pebbles needs to be taken into account.

The direction of future work to explore the question is clear;

… models of water delivery and planet growth need to synchronize the timing of earliest planetesimal formation, the mutual influence of collisions and 26Al dehydration, the potential growth by pebble accretion, and the partitioning of volatile species between the interior and atmosphere of growing protoplanets in order to further constrain the perspectives for rocky (exo-)planet evolution.

The paper is Lichtenberg et al., “A Water Budget Dichotomy of Rocky Protoplanets from 26Al-Heating,” Nature Astronomy Letters, 11 February 2019 (abstract). Thanks to John Walker for helpful information regarding this story.

The embryos used in these simulations are, says the paper, “…of initially lunar mass, M = 0.0123 MEarth,” and this embryo “is placed randomly between specific inner and outer bounds within the protoplanetary disk… It starts accreting solids (planetesimals) and gas, and may migrate in the type I and II regime, depending on the embryo mass and physical structure of the disk at a given orbit.” An embryo like this could also be called a protoplanet.

In this era a supernova evidently occurred in the astronomical neighborhood, depositing ²⁶Al and other elements into the mix.

I recall there was a suggestion that the origin of the aluminium-26 was more likely to be as a result of the Solar System forming in the bubble from a Wolf-Rayet star. Does the Wolf-Rayet hypothesis still hold up? Admittedly Wolf-Rayet stars do tend to go supernova, so perhaps both processes played a role.

The discovery of aluminum 26 last year resulting from the collision of two stars provides the first observation of a point source for aluminum 26 and may provide a means to find other localized sources. If the theory holds water (pardon the pun) we may have at hand a way to zero in on likely planetary systems.

This is a fascinating idea. I had read years ago that radioactive aluminum (26Al) was what caused the cores of large asteroids to melt back in the early formation of our solar system, so this theory makes sense.

Consider the importance of non-biologically beneficial, even potentially harmful radioactive element isotopes like 26Al and Uranium-238. 26Al (half life 717,000 years) is likely responsible for Earth having the right amount of water in its early history. Plus it decays into the biologically vital element magnesium. 238U (half life 4.47 billion years) being heavy would have partly settled deep inside the Earth, where the slow decay of it and its many radioactive daughter elements would have kept the Earth’s outer core liquid. Without a partially liquid core we would by now have no protective magnetic field, no tectonics, no mountain building and no remaining surface water. Radioactivity is vital for there to be life bearing planets.

But a half life of 717,000 is a relatively short period in the development of a solar system, which ours took 100 million years. After less then 3 million years it is down to 6.25% or 1/16 the amount that is only 3% percent of the age of solar systems massive disk of material that surrounded our sun. So if the AL26 breaks down the H2O will it reform after the radioactivity decines?

We still have much to learn about how planetary systems form Michael. The evidence favors rapid formation of the planetary embryos mentioned in the paper under discussion. Planetesimals must have come together quick enough for there still to have been enough heat from radioactive decay (and not just from Al-26, certainly) to cause their interiors to melt. We know this happened here in our system from studying meteorites. While it’s true that compared to 100M years 717K years is short, all that is needed for this theory to work is that the formation of these planetary embryos happen quickly. After they have formed it can take as many millions of years as needed for them to coalesce into planets.

The 26Al isn’t breaking down the H2O — it’s vaporizing it by heating up the interior of these planetesimals, which causes them to degas the water and leaves behind drier building blocks for planets. The H2O likely wouldn’t re-penetrate these rocks post-dessication

Interesting hypothesis. If this means formation must be in a dense star cluster or by happenstance near a supernova, then we need to know what sort of frequency this implies. Is there any way to determine AL-26 distribution in space directly or via proxies? If not, how do we test this theory?

But most stars very likely form in dense star clusters, so look on the bright side. Also 4.5 BYA the SN rate would have been greater than it is today. (OTOH, we now know what it takes to produce the heaviest elements; a massive close binary in which both stars produce neutron star SN remnants, which then spiral together to produce a kilonova.)

The decay of Al-26 emits gamma rays and X-rays, so there’s a good chance that it’s abundance might be measured in SN remnant blast debris.

If the Earth’s solid surface had no relief (was totally smooth) the 1 global ocean we have today would be about 1500 km deep everywhere. The Earth could have essentially been a waterworld in the very distant past.

One study (Zeng, Harvard) of 4,000 exoplanets suggests that up to 35% are water worlds of up to 50% water (Earth is around 0.02% I think). That would greatly reduce the number of life bearing planets wouldn’t it (no carbon cycle, so no stable climate as Paul says)? So together with the need to be in the habitable zone are we beginning to zero in on the number of planets with life similar to our own? Would it even be 1% of the total? I tend to doubt it. Would it be 0.1%? I suppose we could think of the solar system as having 2 potential life bearing worlds (Earth and Mars) for a considerable length of time but we must have had a significant amount of Al26 in the system to produce 4 rocky planets. So this is getting tricky for extraterrestrial life. Possibly there are huge life deserts in the galaxy where Al26 is lacking and only scattered oases of life? How would this affect the spread of a space faring species? If the trip to the next habitable world is 1,000 light years versus 10-50 light years what are the chances it will be successful?

It isn’t just a supernova, lots of systems come close to those,
it’s the timing of an adjacent supernova.
This is why the tragedy of Kepler’s inability to spot Earth twins in
the HZ of K and G type stars, continues to irritate me . We’ve found Super Earths in the HZ of those type of stars with Kepler, but we are assuming Earth sized terrestrials common form in the HZ of K and G stars, because?. This hypothesis on water worlds makes me even more doubtful that the missing Earth Twins actually exist. Not only does the volume of ocean on terrestrials matter, but also how many other metals are stripped from the protoplanetary disk when in close proximity to supernova. Yes some materials may arrive from
a nearby super nova, but what if the net effect is to lower the final masses of any terrestrial planets?. The abundance of Super Earth’s in
Kepler’s catalogue may indicate that w/o a supernova disturbance at the right time, a solar system tends to create vastly more super earths
than Earth’s. Before anyone mentions the Trappist system , i’d like to point out that it is an outlier result and it is Orbiting a Red Dwarf and a damned small one at that.

I agree with much of what you say. Yes, the Kepler results where disappointing in the haul of earthsized planets, but that might be due to the limitations of the instrument coupled with the finding that most stars are more active than was expected. In nature the normal trend is that smaller objects are more numerous than larger, while in space smaller is always harder to find than larger. So on the basis of those fundamental principles we might still find that Earthsized planets outnumber super earths.

Certainly a novel concept to me and the irony that radioactive materials may have been essential, indirectly, for the formation of life is oddly satisfying to a pro-nuke guy like me.

Yet a water world would not necessarily be devoid of carbon and other elements. Perhaps volcanic activity could breech the high pressure ice layer to release minerals into the ocean. Perhaps enough meteoric materials can reach the ocean surface to enrich the upper layers of water to somehow create an environment suitable for bio-genesis. It would be a stretch based on what we know about life but what we don’t know is certainly a much larger set of knowledge.

And one more possibility to enrich the the water, certainly there will be significant storms on such planets with copious lightning. Depending on the atmospheric composition, a lot of biologically important compounds could be formed. Of course, the atmosphere must be a repository of needed elements.

But what process will act as a carbon sink on such worlds? That is why plate tectonics is needed. Most of the CO2 from Earth’s atmosphere is now rock. On a water world, the CO2 would be buffered in the ocean, but unable to be sequestered. Dr. Ramirez has hypothesized a CO2 clathrate model that might work under constrained conditions but does not look like a universal solution.

>On a water world, the CO2 would be buffered in the ocean, but unable to be sequestered.

Nope. Some ocean organisms are, on Earth, a significant source of carbon sequestration in the deep ocean. They secrete CaCO3 in their cell walls; after death, they sink and form bottom sediments. There’s lots of biogenic limestone and chalk in the world.

Carbonate compensation depth
The carbonate compensation depth (CCD) is the point in the ocean where the rate of precipitation of calcium carbonate is balanced by the rate of dissolution due to the conditions present. Deep in the ocean, the temperature drops and pressure increases. Calcium carbonate is unusual in that its solubility increases with decreasing temperature. Increasing pressure also increases the solubility of calcium carbonate. The carbonate compensation depth can range from 4,000 to 6,000 meters below sea level.

Normally, chalk forms in warm shallow seas. These will not be common on water worlds. Instead, they will have very deep, ocean, much deeper than the 4-6 km carbonate compensation depth. We even expect various types of dense ice to separate the liquid column from the underlying crust. This suggests to me that CaCO3 will not sequester on water worlds like the chalk formations we see on Earth. [But I am not a geologist, so I may be wrong.]

At this point, I’ll say I’ve made my point, that it’s fallacious to claim that an ocean world can’t have a carbon cycle. What’s very clear is that whatever carbon cycle it does have is going to rather different from the one on Earth. It looks like a good problem for an exogeologist to work out in more detail. It’s all rather more complicated than just calcium carbonates, which was my first thought for reply.https://en.wikipedia.org/wiki/Oceanic_carbon_cycle

As to specifics, calcium would likely play a different role. It could still cause downward transport, just not all the way to the bottom. I’d also expect the deep waters to be highly anoxic, favoring entirely different processes, possibly even kerogen formation.

Dr. Ramses Ramirez has proposed a CO2 regulation via CO2 clathrates for worlds in the HZ with polar ice caps. The clathrates sink to sequester the carbon. They can also release the CO2 to replenish the atmosphere too. It is a little complex, requires a “Goldilocks” condition, is unproven, but it is a possible mechanism to regulate CO2, and therefore climate, on a water world.

Granted that Al 26 would tend to adversely affect volatiles in planetesimals, but there is another consideration with respect to the Earth. This piece of real estate also had an early collision with a body that caused formation of a very large moon. That must have had an effect on its chances to be a water world today. Reflecting on that, it might be that rocky worlds still have a lot of water in early formation. So if we have had some concerns about environments around M-type stars, maybe there is reason to reconsider or re-calibrate our notion of how much water a supposedly rocky world would start out with.

“Two interesting articles on different types of planets, the first one is dealing with habitability of carbon-enriched rocky exoplanets. Basically a graphite covered world, but what would impacts and plate tectonics or oceans due to such planets? We are finding very quickly just how common and recent impacts have been on earth and how they can upset the atmosphere, geology (As in plate tectonics), oceans and ice ages. We find the earth as a dynamic planet and the water, dry and carbon planets will have similar dynamics.

Mineralogy, structure and habitability of carbon-enriched rocky exoplanets: A laboratory approach.
Carbon-enriched rocky exoplanets have been proposed around dwarf stars as well as around binary stars, white dwarfs and pulsars. However, the mineralogical make up of such planets is poorly constrained. We performed high-pressure high-temperature laboratory experiments (P = 1−2 GPa, T = 1523−1823 K) on carbon-enriched chemical mixtures to investigate the deep interiors of Pluto- to Mars-size planets the upper mantles of larger planets.
Our results show that these exoplanets, when fully-differentiated, comprise a metallic core, a silicate mantle and a graphite layer on top of the silicate mantle. The silicate mineralogy (olivine, orthopyroxene, clinopyroxene and spinel) is largely unaffected by the amount of carbon. Metals are either two immiscible iron-rich alloys (S-rich and S-poor) or a single iron-rich alloy in the Fe-C-S system with immiscibility depending on the S/Fe ratio and core pressure. Graphite is the dominant carbon-bearing phase at the conditions of our experiments with no traces of silicon carbide or carbonates. If the bulk carbon content is higher than needed to saturate the mantle and the core, graphite would be in the form of an additional layer on top of the silicate mantle assuming differentiation. For a thick enough graphite layer, diamonds would form at the bottom of this layer due to high pressures.
We model the interior structure of Kepler-37b and show that a mere 10 wt% graphite layer would decrease its derived mass by 7%, suggesting future space missions that determine both radius and mass of rocky exoplanets with insignificant gaseous envelopes could provide quantitative limits on their carbon content. Future observations of rocky exoplanets with graphite-rich surfaces would show low albedos due to the low reflectance of graphite. The absence of life-bearing elements other than carbon on the surface likely makes them uninhabitable.”https://arxiv.org/abs/1807.02064

The second article deals with tidally locked planets and makes the good point that the majority of habitable worlds will be in that state, simply because the number of M and K dwarfs suns in the galaxy. This is another area that has not been given much thought to what would impacts and plate tectonics or oceans due to such planets. The nature of their orbits would put them in a high probability from comet and asteroid impacts due to the short orbit period and nearness to their sun. This alone would cause greater tectonic activity and change both the geology and oceans over long time periods. And I bet the impacts on the planet’s surface would also form repeating geologic patterns that would affect atmospheric dynamics and oceanic currents. ;^{

The Bizarre Planets That Could Be Humanity’s New Homes.
What would human civilization look like on a tidally locked world?
CHARLIE JANE ANDERS
FEB 13, 2019

If you have not figured out how tidally locked planets could have a repeating geologic pattern from meteors, it is simply because our meteor showers peak at 2 AM. The earth is at that time going directly into the meteor stream as it orbits around the sun. On a tidally locked planet it does not rotate as the earth does in 24 hours so the 2 AM location is always at the same location on the planet! Now the question I have for the experts is whether larger bodies (Asteroids, Comets) would also have a tendency to impact around that location? The point being that besides the effect on the geology of the nighttime point at 2 AM there would be an antipolar effect on the daylite side at the 2 PM or about 30 degrees from high noon, this could cause large volcanic extrusions at that location as we have seen at the opposite side of the earth from large impacts.

Now what you have to remember is especially for red dwarfs (75% of stars) that these close in planets travel at a much higher pace and the asteroids and comets that orbit that close will be confined to a much smaller area so impacts will be much more common. If this holds true for the larger impacts then there will be large impact basins and lava outflows plus mountain building and dynamic plate tectonics around the 210 degree position from high noon. At the opposite side of the planet at the 30 degree position from high noon there will be chaotic terrain, large lava extrusions like the columbia basin and areas of the release of natural gas and petroleum.

Now since taking leave of my long range telescope, what do you think? Could there be different patterns associated with these types of planets? :-}>

If your hypothesis is correct, it should be very evident on tidally locked moons in our system, including our own Moon. There are many tidally locked moons around the gas giants, all of which should show asymmetric cratering. I am not aware of any such cratering patterns, but maybe you have?

Ah, but you our forgetting, they all orbit around their planet and are not tidally locked to the sun. The meteor streams and other larger bodies orbit around the sun and that is why they occur at the same time each year when the earth crosses the orbit of the original comets path. A prime example is the Perseids meteor shower in August each year, Earth will pass through the path of Comet Swift-Tuttle from July 17 to Aug. 24, with the shower’s peak — when Earth passes through the densest, dustiest area — occurring on Aug. 12-13. https://cdn.mos.cms.futurecdn.net/Bz6puv3aXvkmYBGJBYSARg-650-80.jpg

The only planets close to tidally locked to the sun are Venus and Mercury, but Venus actually has a retrograde rotation and Mercury has a spin-orbit resonance of 3:2. So there are no example in our solar system that would have a detectable pattern of impacts from tidal locking to the sun. The best example is the tightly packed Trappist 1 system, that has an M dwarf Sun the size of Jupiter but with a mass 84 times it.https://www.jpl.nasa.gov/spaceimages/images/largesize/PIA21428_hires.jpg

The furthest planet out in that system, h is 6.25 times closer then Mercury is to our sun and planet b is 34 times closer.

Nice conjecture Michael, and good answer to Alex’s objection.
The leading hemispheres of completely tidally locked rocky surfaced exoplanets would have to suffer both more impacts and impacts of greater velocity than the trailing sides.

Consider a comet, like Swift Tuttle, but orbiting co-planer in our system. Now consider tidally locked Mimas around Saturn orbiting at ~14 km/s (and Saturn is orbiting at ~10 km/s). At Saturn, the comet is also traveling ~ 6 km/s towards the sun. When Mimas is moving to3wards the sun, it is traveling faster than the comet debris and will not be impacted on its “rear” hemisphere except due to Saturn’s orbit. When traveling away from the sun, Mimas will be impacting teh debris on teh “forward face” at ~20 km/s. The same faces will be impacted if teh comet is outward bound from the sun. This looks highly asymmetric to me and should result in Mimas’ forward facing hemisphere being impacted more frequently and with greater velocity than teh rearward facing hemisphere. Yet there is no evidence of cratering asymmetry. When including Saturn’s orbital velocity, when Mimas is “in front of” Saturn, it should be more exposed than when behind and shielded by Saturn. Yet its hemisphere facing Saturn is no less cratered than its hemisphere facing away from Saturn.

[I chose Mimas because it has a solid surface and no subsurface ocean to remove craters like Europa or Enceladus and because the comet would be orbiting relatively slowly out near Saturn, creating a greater velocity difference between it and Mimas.]

I see your point and a better example would be Jupiter’s moons, but as in Comet Shoemaker–Levy 9 anything coming close to the large gravity field of these planets would have their orbits changed. You are then dealing with three body problem dynamics so impacts would be scattered. But thank you, now you have me wondering if we might see some of the unusual shapes of moons around Saturn, saucer shaped planets and raised equatorial mountains, possibly asymmetric brightening as in Iapetus. Could any of this be picked up in the photometric dips when the eclipses occur or when the planets reflect the stars light. What about dense asteroid belts between planets as in Trappist 1 and I’m still waiting to hear if anyone has tried to catch the UV reflections off planets when the bright UV flares take place???

Regarding tidally locked planets in systems like Trappist-1, there would appear then to be a couple of unlocking mechanisms.

1. The idea that all the impacts would hit at the same spot would cause a repeated hammering and eventual chance in principal axes of inertia,be they as small as they are. That would cause some sort of drift or bobbing into a new latitude and longitude of stable orientation. I suppose the unstable motion would have to dissipate.

2.Also, the planets will interact with each other. Witness the Galilean Moons around Jupiter, experiencing successively higher geothermal
and volcanic activity the closer they get to Jupiter. It’s not because they are orbiting Jupiter per se, but that their interaction causes them to twang in their orbits. And I would suspect that if there were only one satellite, Jupiter’s bands and belts would look a lot less chaotic too…

But to some degree, to have a carbon cycle, you need something to stir the pot, right? However, if you are buying a plot, you might need to have some owner’s insurance.

Interesting wdk, Venus has a resonance of orbit and rotation with earth where the same face is facing earth when at inferior conjunction. That is one reason it took so long to figure it’s rotation period, for a long time they thought it was tidally locked to the sun!https://earthhavenlearning.ca/photos/custom/Venus-Cycle-Cesar.jpg
“Venus’ rotation is somewhat unusual in that it is both very slow (243 Earth days per Venus day, slightly longer than Venus’ year) and retrograde. In addition, the periods of Venus’ rotation and of its orbit are synchronized such that it always presents the same face toward Earth when the two planets are at their closest approach. Whether this is a resonance effect or merely a coincidence is not known.”https://nineplanets.org/venus.html

As for the last point – Make sure and get mineral rights to the plot – There’s going to be a lot of heavy metal lying around and buried.

That’s something that struck me from the Mars rovers images, all those nickel/iron meteorites they keep finding! If a base is ever established they should send drones out to find them and return them to the base. They can be ground up and used to make metal parts in a 3D printer instead of hauling it from earth!

That is a key element in making the Earth the way it is. It keeps Earth’s axial tilt stable (seasons). It keeps our seas well stirred (tides). Back when the great collision between a Venus sized proto Earth and a Mars sized body merged the cores of both bodies merged to form our magnetic shield producing liquid outer core. If this hadn’t happened our smaller planet would probably be a waterless cross between present day Venus and Mars.

Thank you. The next question is, how often does such a collision scenario happen throughout the Milky Way galaxy and beyond to produce worlds that are not only Earth-sized and have a moon similar to ours, but can also support life?

Moon producing collisions might not be all that rare, considering that at least two of such likely occurred here in our system (Earth and Pluto). But when you string all the factors that you have into your question ljk, well then you have one of the cornerstones of the Rare Earth hypotheses.

The importance of the moon to life on earth is typically huge over-estimated. See J Lissauer, J Barnes, J Chambers; ‘Obliquity Variations of a Moonless Earth’, Icarus, 217 (2011) 77–87.
A large moon is NOT necessary to stabilize a planet’s obliquity with respect to its ecliptic and, as has been pointed out by Barnes and collaborators, can actually be harmful (J Armstrong, R Barnes, S Domagal-Goldman, J Breiner, T Quinn, V Meadows; ‘Effects of Extreme Obliquity Variations on the Habitability of Exoplanets’, Astrobiology, April 2014, 14 (4), 277–291.). In a similar vein, OF COURSE earth would have tides without the moon as the moon only supplies 2/3 of our tides now, with the sun’s gravity supplying the other third. In fact, ONLY solar tides would be MUCH more stable over the life time of the solar system as the tidal force goes as 1/r^3 with the moon’s distance increasing since its formation

Thumbs up, much if not most of RE belief is based on determinism and circular reasoning: ‘this is how we know it here, so this is how it has to be’.
The same is the case for Jupiter, once assumed to be a protector of the Earth against impacts. Research has found that Jupiter slings at least as many objects in our direction as it absorbs.

Is this yet more evidence that habitable worlds as we understand them will be rare or very rare? It’s starting to look that way to me. The presence of significant Al26, a large moon, a core which is mostly molten iron but with significant radioactive components to produce heat to keep the core molten, a significant amount of water but also land area, in the habitable zone of its parent star etc. The list appears to be a lengthy one. Will we find other worlds similar to our own in our region of the galaxy. So far the bias has been to larger worlds that are more easily detectable but that will continue to change. I look forward to being able to learn about rocky planets similar to our own but the list may remain meager for a long time. One of the most interesting areas of study will be where does life arise that we didn’t expect and how complex can it become over time?

“Is this yet more evidence that habitable worlds as we understand them will be rare or very rare?”

I think not. It is one more barely understood factor among a multitude of factors, known and unknown, that *may* play a role in the formation of planets with certain attributes. Certainly a useful study, however do not be quick to accept it as a universal truth and then extrapolate. Our understanding of planetary formation and evolutionary processes is still quite weak, as is our understanding of the conditions for planetary life.

To re-phrase that final question: what is the rate of evolution in a range of environments which vary from very hospitable (Earth?) to very inhospitable (moons of gas giants outside the so-called habitable zone)? I’m using the planets and moons of our own solar system because we can study them best but eventually I hope we have data from nearby star systems (Proxima Centauri b, Trappist-1e, GJ667 C f, GJ667 C e and so on).

Thanks for your comment Ron. I think you are right on all counts. We don’t know much at all yet about what defines the outer bounds of an environment that would allow life to arise. I suspect it will be quite wide indeed. A source of nutrients, water, and energy should suffice to at least allow microbial-like life to arise given enough time. That is why studies of Mars and the gas giant moons known to have water under icy shells will be so important. In other solar systems there are so many variables to take into account including the type of star, how active it is, distance of the planet from the star, whether the planet has a core which will generate a protective magnetic field, whether land and water are both present, tectonic activity and on and on. It’s a great time to watch the data role in!

This idea is not supported by the physics: “Al inside the planetesimals can create heat that can evaporate much of the initial water ice content before accretion occurs. Dense star-forming regions are more likely to produce planets that manifest these latter outcomes.” Planetesimals are only 1 km so they are too small and their the heat from radioactive decay is modest.

“Small bodies have a higher area to volume ratio values, so they loose heat faster than large bodies and heat from the decay of Uranium, thorium, and potassium 40 is modest.” P. 300 The New Solar System.

Earth would not have any water if it was lost before their were any protoplanets. All our water would have to have come from comets, but none from accretion. All the water worlds would be around super Earths from M class stars. There might be more water available for super Earths around larger stars since they are born from larger clouds of gas and dust. With more gravity there are more collisions in a heavy bombardment period.

Then what accounts for the fact that we have nickel-iron and stony-iron meteorites? The existence of such prove at the very least that some now shattered asteroids had once melted and had stayed molten long enough for the heavier metals to sink to their cores.

Also, many of these planetesimals must have been far larger than just 1km. Some likely would have grown to at least the size of our largest asteroid Ceres. Were did 1km come from?

And this wouldn’t have cooked out all of the water, just enough to have left us with the “goldilocks” amount.

One shouldn’t take Kepler’s not finding an “earth twin” as being ANY proof as to their non-existence. Given the higher than expected noise level from sunlike stars, and the need to see at least three transits (two years in the very best of cases), this is not really unexpected. The best estimates I’ve seen for eta-earth is about 0.20, with error bars at least half that size. By way of comparison, less than 0.02 of the earth’s humans are taller than 6’3″…..

I agree completely with this comment Coolstar, as it is a better stated equivalent to part of my above reply to Admiral_Ritt. (Also, I’m 6’5″, part of what makes me used to being an outlier ;)

I also appreciate your first comment in this fascinating discussion re the importance of a moon for life being overstated. Nicely informative counterpoint. Ok then, I’ll concede that having a large moon might not be a requirement for habitability. But I still think that in our system our having had the very large impact that also happened to create our Moon would have been a very important contributor to Earth’s long-lived habitability. It enriched our planet’s stockpile of U238 and explanded our planet’s partially molten core, both in size and duration.

@ Bruce Mayfield I was surprised myself at the height distribution of humans! (easy to remember the average, hard to remember the standard deviations.?….). I fail to see how the earth’s supply of U238 and thus the long term size and temp of the core could have been increased MUCH by the creation of the moon as the impactor probably had only about 10% of the earth’s total mass. Am I missing something obvious?

Consider that uranium is 70% denser than lead. Proto-earth and the great impactor are each mostly molten at times prior to their collision, so most of their U would have been consentrated in their cores. After impact both pre impact cores combine inside the Earth. Proto-earth was about the mass of Venus, so the upgrade in the Earth’s total supply of the heaviest elements increases by more than 12% at the least, I would think. Plus heat from the great impact itself melts the Earth and helps get this slow release nuclear fuel (U238) down deep where it’s needed.

Most of the biosphere runs on solar power, but internally the Earth’s geological power source (after the heat from impacts and gravitational contraction runs out) comes from the slow release of energy stored inside atoms created from the collision of Neutron stars. I’m glad Earth’s nuclear fuel tank was topped off. ;)

I still don’t buy this dried out planetesimal theory. Ceres is not exactly bone dry either. The radioactive decay in Ceres core did not evaporate it’s water and it’s a much larger body. It’s the size of the planet that also determines the water since a large gravity makes it harder for it to escape and it’s distance from the star. Carbon, oxygen and nitrogen are also metals. Limit size of cloud or star and we might have less of these and less water than the potential water in larger clouds.

I also don’t agree with the idea that a smaller cloud of gas will make the water worlds more likely. We will find out for sure in the next ten years with the exoplanet spectroscopy of the more powerful Earth based telescopes.

I only mentioned Ceres as a remaining example of the size planetesimals attained. Yes it isn’t dried out, but it formed out near or beyond our system’s snow line, much further from the sun than the planetesimals that built up the Earth. But we can’t say exactly how the Earth formed since we weren’t there to witness it, so maybe we can have a friendly agree to disagree on this one?

Makes you wonder about planets around M-Dwarf stars. Because M-Dwarf stars flare, many worry that close-in terrestrial planets would lose their atmospheres over time.

But if those terrestrial planets start as water worlds, losing atmosphere may be a good thing (good in terms of making a world more Earth-like). If M-Dwarf stars cause water worlds to lose volatiles, then maybe over time that excess water will be lost to space, and you’ll end up with a more Earth-like world. Interestingly, because hydrogen will be lost faster (being so much lighter than oxygen), over time the world’s remaining atmosphere maybe come relatively saturated with oxygen, even without any photosynthesis.

So this may be a pathway to having oxygen enriched atmospheres without having any life.

Good point Eric, the only problem is the oxygen could also form CO2, this is why we need the large scopes and JWT. I’m sure we will all be surprised at what constitute the atmosphere of many planets, since both nature and physics has been so far limited to our solar system planets.

This article says that it is the radioactive decay in the planetesimals is what evaporates the water which is what I don’t agree which includes the inside of the planetesimal not just the surface, the snow line only applies to the surface of the body.

I don’t disagree with the idea that that water world super Earths are more likely to form in smaller gas clouds. I just don’t see any concrete evidence to support the idea. Aluminum 26 does not determine how much oxygen and hydrogen a gas cloud can have which are needed to make water.

Beside the issue of how much water is supposedly burned off accretion disks by Al26, how do we get quantitative about distance from a supernova and the amount of Al26 deposition in the accretion material.
For example, shall we say the early sun’s collapsing disk is ten light years away or one hundred from the original supernova. If it is 100 light years, then a number of other proto suns and accretion disks would experience the same Al26 enrichment. Plus there would be bands of more and less. But even in these cases, intervening events such as the energy of collisions would have an effect as well. Local circular velocities around a less massive sun would be higher in a habitable zone than it would be for a G such as the sun. Collision energies would not be identical.

Since magnesium is the stable end product of Al26 decay the amount of Mg in a system, star or planet might help answer wdk’s questions, but only if the baseline amount of Mg produced by fusion in stars is known.
Note what Wikipedia says re Mg:

“Magnesium is the ninth most abundant element in the universe.[5][6] It is produced in large, aging stars from the sequential addition of three helium nuclei to a carbon nucleus. When such stars explode as supernovas, much of the magnesium is expelled into the interstellar medium where it may recycle into new star systems. Magnesium is the eighth most abundant element in the Earth’s crust[7] and the fourth most common element in the Earth (after iron, oxygen and silicon), making up 13% of the planet’s mass and a large fraction of the planet’s mantle. It is the third most abundant element dissolved in seawater, after sodium and chlorine.[8]

Even though it wasn’t mentioned in that paragraph, some percentage of Mg comes from the decay of Al26. An ultimately testable prediction from this would be finding dry systems with enhanced Mg levels. The larger the Mg level, the closer to the SN perhaps? There would also be other elemental evidence as well very likely.

Bruce D. Mayfield,
Thanks for the help and research on this. It gives a little hope that maybe someday soon this planetary formation feature will be characterized better. I went back to the article above and the
abstract. A lot of runs, perhaps Monte Carlo style, but there might be a clue already about how far away the early sun and circum-stellar disk had to be to get is Al26 content.
If I remember correctly (doubtful), SN activity and the resulting shocks or density waves could be responsible for the collapse of a lot of clouds which form into suns and surrounding disks. The Kepler data so far might be tentative toward high content water worlds, but probably not a closed case.
Also, a lot of our discussion about habitable worlds seems to bounce between the idea of a thermal zone where they can be found and a set of circumstances that can produce a world identical to our own.
I keep thinking that habitability can be achieved by other means if you spray yeast onto a primeval planet. We are still guessing at recipes, but haven’t seen any bake sales yet.

I reread Paul’s article in an attempt to understand your objections Geoffrey. I get your point that the planetesimals would need to be of sufficient size for them to heat up, and I see that the 1 km planetesimal size was mentioned by Paul. The only thing that Al26 has to do with water is that heat from its decay (into Mg) would help drive light volatiles like H2O to the surface of planetesimals. (Heat would also be supplied by collisions.) In the inner parts of a young system the stellar wind would then tend to strip the H2O away. (Stars are more active in their youth, flairing and stellar winds are greater.)

The reason smaller stellar system forming gas and dust clouds would tend to produce water worlds would be (according to this theory) that smaller clouds wouldn’t be near as likely to produce nearby SN which could provide isotopes like Al26 which can help dry out the building blocks of newly forming planets.

It would be getting more buzz if it wasn’t hid behind a tall pay wall.
Here is the paper’s abstract:

“Measures of exoplanet bulk densities indicate that small exoplanets with radius less than 3 Earth radii (R⊕) range from low-density sub-Neptunes containing volatile elements1 to higher-density rocky planets with Earth-like2 or iron-rich3 (Mercury-like) compositions. Such astonishing diversity in observed small exoplanet compositions may be the product of different initial conditions of the planet-formation process or different evolutionary paths that altered the planetary properties after formation4. Planet evolution may be especially affected by either photoevaporative mass loss induced by high stellar X-ray and extreme ultraviolet (XUV) flux5 or giant impacts6. Although there is some evidence for the former7,8, there are no unambiguous findings so far about the occurrence of giant impacts in an exoplanet system. Here, we characterize the two innermost planets of the compact and near-resonant system Kepler-107 (ref. 9). We show that they have nearly identical radii (about 1.5–1.6R⊕), but the outer planet Kepler-107 c is more than twice as dense (about 12.6 g cm–3) as the innermost Kepler-107 b (about 5.3 g cm−3). In consequence, Kepler-107 c must have a larger iron core fraction than Kepler-107 b. This imbalance cannot be explained by the stellar XUV irradiation, which would conversely make the more-irradiated and less-massive planet Kepler-107 b denser than Kepler-107 c. Instead, the dissimilar densities are consistent with a giant impact event on Kepler-107 c that would have stripped off part of its silicate mantle. This hypothesis is supported by theoretical predictions from collisional mantle stripping10, which match the mass and radius of Kepler-107 c.”

“A physically realistic stellar wind model based on Alfvén wave dissipation has been used to simulate the wind from Barnard’s Star and to calculate the conditions at the location of its recently discovered planetary companion. Barnard’s Star b experiences much less intense wind pressure than the much more close-in planet Proxima~b and the planets of the TRAPPIST-1 system. The milder wind conditions are more a result of its much greater orbital distance rather than in differences in the surface magnetic field strengths of the host stars. The dynamic pressure experienced by the planet is comparable to present-day Earth values, but it can undergo variations by factors of several during current sheet crossings in each orbit. The magnetospause standoff distance would be ∼\,20−40\,\% smaller than that of the Earth for an equivalent planetary magnetic field strength.”

Volatiles like methane and ethane would also tend to boil to the surface of Al26 heated bodies Laura. But seas of ethane nine times more common than water? That seems very far fetched, since the elemental abundance of oxygen is about three times that of carbon. H2O, very common, C2H6 much less so.

But the best example we have is Titan and both Venus and Mars have mostly carbon dioxide atmosphere. I would say that from planets 3/4 earth mass to 3-4 earth mass beyond the snowline would have both. The water would be our equivalent of granite or if you like slow moving glaciers and the ethane oceans would be the driver as water is here in lubricating the plate tectonics. It would be interesting to see if the same thing would occur as the mid oceanic ridges and their spreading with ice volcanic being the prime mover. Worlds where ice is the rock and magma and ethane is the liquid,
sounds like life might developed since carbon would be plentiful. What would plastics be like at those low temperatures or any other material that would have a change of state at -290 degrees fahrenheit?

I understand it is not directly related with the atomic abundances itself but in the abundance of scenarios that can develop such seas, that are more plausible around red stars (which are more abundant).

Thank you, fascinating article. So if the most common life is based on the liquid ethane, the reason they have not landed on White House lawn is they would boil into a glob! This will make for some very interesting discussions on ethane life and their technology would all be based on superconductors! ;-)

I’ve just found in FB the pdf of a popularization paper about “Diving into exoplanets”. Interesting; the journalist has also interviewed other scientists asking for their opinion on the paper. Here is the link:

Earth’s future detectability depends upon the trajectory of our civilization over the coming centuries. Human civilization is also the only known example of an energy-intensive civilization, so our history and future trajectories provide the basis for thinking about how to find life elsewhere.

This special issue of Futures features contributions that consider the future evolution of the Earth system from an astrobiological perspective, with the goal of exploring the extent to which anthropogenic influence could be detectable across interstellar distances.

This collection emphasizes the connection between the unfolding future of the Anthropocene with the search for extraterrestrial civilizations. Our rate of energy consumption will characterize the extent to which our energy-intensive society exerts direct influence on climate, which in turn may limit the ultimate lifetime of our civilization.

If the answer to Fermi’s question is that we are alone, so that our civilization represents the only form of intelligent life in the galaxy (or even the universe), then our responsibility to survive is even greater. If we do find evidence of another civilization on a distant exoplanet, then at least we will know that our trajectory can be managed. But as long as our searches turn up empty, we must stay vigilant to keep our future secure.

Comments: Introduction to the special issue on the Detectability of Future Earth, published in Futures (volume 106, pages 1-44)

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

If you'd like to submit a comment for possible publication on Centauri Dreams, I will be glad to consider it. The primary criterion is that comments contribute meaningfully to the debate. Among other criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language, and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations. A long form of the policy can be viewed on the Administrative page. The short form is this: If your comment is not on topic and respectful to your fellow readers, I'm probably not going to run it.